One thing that makes this interesting is the information difference. If this wasn't second time around for Da-ae, and she was actually hanging out with him because she was either going out with him or aiming to do so, she might be shocked and upset by this development--she'd be all "You never told me you were going out with someone!"
But as things are, that's not why she's hanging out with him (as far as she's figured out yet, anyway), she has memories of this manipulative creepy girl being manipulative and creepy, and also from her perspective whatever was happening between him and that girl was over in high school and basically unimportant except for The Incident. So whatever happens, her reactions will be coming from a totally different place than whatshername can reasonably expect.